The present invention relates to methods for forming semiconductor devices. In particular, the present invention relates to methods for forming dielectric layers on semiconductor devices.
Dielectric layers on semiconductor devices are used to isolate or protect other layers. Currently silicon dioxide, silicon nitride and silicon oxynitride are the most commonly used dielectric layers.
One type of dielectric layer is a thermally-grown silicon dioxide. Thermally-grown oxides are formed by reacting silicon on the substrate with oxygen at high temperatures. Field and gate oxide layers are examples of thermally-grown oxides.
Another type of dielectric layer is a deposited dielectric layer. Deposited dielectric layers include interlevel dielectric layers such as boro-phosphorous silicate glass (BPSG), inter-metallic dielectric layers and passivation layers. Typically, deposited dielectrics are formed with a chemical vapor deposition (CVD) system. Common CVD systems include plasma enhanced chemical vapor deposition (PECVD) and low pressure chemical vapor deposition (LPCVD). In chemical vapor deposition, reactants in a gas or vapor react to form a dielectric material which is deposited on the substrate. To form a deposited silicon dioxide layer, typically a mixture of nitrous oxide (N.sub.2 O), silane (SiH.sub.4) and tetra-ethyl-ortho-silicate (TEOS) are used. To form a nitride layer, such as Si.sub.3 N.sub.4, the reactant substances include silane and ammonia NH.sub.3. To form a combined oxynitride, a mixture of silane, nitrous oxide and ammonia is used.
A concern with metal-oxide-silicon field-effect-transistors (MOSFETS) is the need to maintain stable and reliable threshold and breakdown voltage values. It is desired to improve the processing steps used to form the dielectric layers to help obtain stable and reliable threshold and breakdown voltage values for MOSFETS.